1. Field of the Invention
The invention relates generally to delivering fluids to a patient and, particularly, to closed-loop flow control systems and methods for delivering medical fluids to a patient at a controlled delivery rate.
2. Description of the Prior Art
A variety of fluid delivery systems are currently being used in the medical field for delivering medical fluids (containing medication, nutrition, saline, and so on) to human and veterinary patients. It is often desirable to administer such medical fluids at relatively precise delivery rates. In some cases, the rate of delivery may be exceptionally important. In recent years, it has also been found to be advantageous to use various types of infusion pumps to administer medical fluids automatically, over extended periods of time. A typical infusion pump delivers the medical fluid into the patient's venous system using a delivery channel which usually comprises an administration tube (e.g., a polyvinyl chloride tube) connected to the patient using some form of catheter, needle, or the like.
Heretofore, infusion pumps and similar devices known in the art have typically not provided closed-loop flow control to achieve precise delivery rates. Rather, flow control has been open loop because actual flow rate information has not been used in controlling the infusion pump. A typical accuracy of such systems, in terms of flow rate, is normally no better than about +/−5%, and requires relatively sophisticated (and costly) mechanical components and tight material/geometry controls (e.g., of the tubing) to achieve that rate. In fact, ambulatory pumps typically achieve accuracies of +/−6-8%. Further, non-ambulatory pumps often do not achieve a five percent accuracy range at low flow rates or over longer time periods due to modification of the tubing material over time. For example, a typical peristaltic type pump requires repeated deformation of the administration tube. This deformation process changes the elastic recovery properties of the tube, resulting in changes in the volumetric output of the pump over time. One volumetric pump available from the assignee of the present application has a specified rating of +/−5% at 1-1200 ml/hr and +/−10% at 0.1-1 ml/hr. Another pump available from the assignee of the present application has a rated accuracy of +/−5% for the first 24 hours of use and +/10% thereafter.
While the foregoing accuracy ranges may be acceptable for some uses, greater accuracy is desirable for other uses. In some prior art systems, the pumping mechanism associated with the infusion pump is monitored and controlled, but the actual flow of fluid in the administration tube is not. For example, commonly assigned U.S. Pat. No. 5,533,981 describes a syringe infusion pump having a sensor for detecting the position and capture of a syringe plunger for use in controlling the dispensing of fluid from the syringe. Commonly assigned U.S. Pat. No. 6,078,273 discloses a variety of known infusion pump systems such as, for example, roller pump systems, peristaltic-type systems, valve-type systems, and motor driven systems. Further, commonly assigned U.S. Pat. No. 5,482,841 discloses a volumetric-type infusion pump. An example of an ambulatory infusion pump is a pump sold under the mark IPUMP by the assignee of the present application. An example of an ambulatory pump may also be found in U.S. Pat. No. 5,993,420.
Some systems have attempted to provide closed-loop control. For example, commonly assigned U.S. Pat. No. 5,533,412 discloses a pulsed thermal flow sensor. In such a system, the fluid is heated by a pulsed heating element. The fluid carries the thermal pulse through a flow channel to two sensor elements spaced apart, downstream from the heating element. The transit time of the thermal pulse between the two sensor elements provides an indication of the fluid flow velocity. Thus, such an approach requires the application of a heat pulse to the fluid in order to determine flow rate information.
Other prior art systems use information generated by positional encoders and decoders associated with a motor shaft to control an infusion pump. For example, the above-mentioned U.S. Pat. No. 6,078,273 discloses an encoder/decoder for use in controlling a medical infusion pump. While such systems reflect improvements in the art, they do not control fluid delivery in view of actual flow rates. In some circumstances, therefore, such systems would not provide as accurate information and tight control based on actual fluid flow rate data.
Sensors, such as positive displacement (PD) flow rate sensors, have been in use for many years and directly detect flow rates. A typical PD sensor includes two complementary rotating elements that, when exposed to a fluid flow, allow a relatively well-defined volume of the fluid to transfer from one side of the sensor to another side of the sensor with each rotation (or partial rotation) of the rotating elements. One advantage of PD sensors is that they support a variety of fluids with substantially equal levels of accuracy. In the prior art, such devices typically measure large fluid flow rates and the requisite level of precision is achieved by conventional precision machining and polishing techniques. In fact, components must sometimes be matched to ensure minimal clearances of the rotating elements and inner housing geometry. Such conventional PD sensors, however, are not well-suited for use in high-precision medical fluid delivery systems. For example, a commercial infusion pump may require the ability to deliver fluids over a wide range of delivery rates (e.g., 4 logs), including very low flow rates. Moreover, conventional manufacturing techniques tend to be expensive and, therefore, are not well-suited for use in manufacturing disposable items.
In recent years, fabrication techniques have developed that allow for the manufacture of micro-fabricated devices. Some of such devices are referred to as micro electromechanical system (MEMS) devices and micro molded devices. One technique for fabricating such devices is referred to in the art as LIGA processing. LIGA (Lithographie Galvanoformung Abormung) was developed in Germany in the late 1980s and translates roughly to the steps of lithography, electroplating, and replication. LIGA allows for the formation of relatively small, high aspect ratio components. Using this technique, a photoresist layer (e.g., an acrylic polymer such as polymethyl methacrylate (PMMA)) is applied to a metallized substrate material. The photoresist layer is selectively exposed to synchrotron radiation (high-energy X-ray radiation) via a mask pattern to form the desired high aspect ratio walls. Thus, the radiation “unzips” the PMMA backbone. The exposed sample is thereafter placed in a developing solution that selectively removes the exposed areas of PMMA. One development solution is 20% by volume of tetrahydro 1,4-oxazine, 5% by volume 2-aminoethanol-1, 60% by volume 2-(2-butpxyethoxy)ethanol, and 15% by volume water. The sample is thereafter electroplated; metal fills the gaps within the PMMA to form a negative image. The PMMA is then removed using a solvent, leaving a metal form for either immediate use or for use as a replication master. The entire LIGA process is described in greater detail in chapter 6, page 341 of Marc Madou, “The Fundamentals of Microfabrication, the Science of Miniaturization,” Second Edition (CRC Press 2001).
LIGA has been identified for use in manufacturing micro-fabricated fluid pumps. It is believed, however, that LIGA-based micropumps have never been made available commercially. Cost is one substantial drawback of LIGA; it is believed that there are relatively few synchrotron devices (e.g., 10-15 devices) in the world. Accordingly, LIGA is fairly limited in its applicability for directly manufacturing low cost devices.
In view of the foregoing, an improved system and method for delivering a fluid to a patient is desired.